Polyolefins, such as various grades of polyethylene and polypropylene, constitute one of the most significant segments of the plastic materials market. Due to the low cost of the monomers (e.g. ethylene and propylene) and the versatility and ease of processing of the materials that can be created by polymerizing them, polyolefins have been widely adopted in a broad range of applications including packaging films and containers, pipes, liners, automotive plastics, wire and cable coatings, and injection and blow molded parts.1 
In homopolymer form, polyethylene can either be highly crystalline and rigid, e.g. high-density polyethylene (HDPE), or of lower crystallinity but soft, tough, and flexible, as in low-density polyethylene (LDPE). The former consists largely of linear polyethylene (PE) chains, while the polymers in the latter are highly branched. Newer grades of polyethylene, such as the metallocene PEs, are manufactured with catalysts and processes that exert more precise control over molecular weight distribution and degree and type of branching than the traditional HDPE and LDPE materials.
Similarly, many grades and types of polypropylene homopolymer (PP) exist in the marketplace. Because of the stereochemistry afforded by the pendant methyl groups along a PP chain, the types and properties of polypropylenes are more diverse than the case of polyethylene. For example, isotactic (iPP), syndiotactic (sPP), and atactic (aPP) polypropylenes are all manufactured commercially. Materials based on iPP and sPP tend to be more highly crystalline and rigid, while aPP materials are soft with low or no crystallinity. As with the case of PE, polypropylenes manufactured using different processes will differ in molecular weight distribution, degree of branching, processing behavior, and physical properties. Both PE and (tactic) PP materials are deemed “semi-crystalline”, since the long-chain nature of the molecules and their entanglement characteristics under both melt and solution conditions thwarts complete crystallization. While the solid state morphology of semi-crystalline polyolefins can vary widely and the crystals can adopt various forms, generally a two-domain structure exists in which polymer chains connect and mechanically engage crystalline domains, which are separated by amorphous domains.
Olefinic monomers can also be copolymerized via a variety of chemical processes to create random or statistical copolymers. For example, copolymerizations of ethylene and propylene are used to make a rubbery material known as ethylene-propylene rubber (EPR). Small amounts of higher alpha-olefins, such as 1-hexene, or 1-octene, are also copolymerized with ethylene to create families of soft elastomeric and plastomeric materials with a broad range of applications.
While block and graft copolymers have been commercialized in other families of polymeric materials, most notably the styrenic block copolymers (SBCs) prepared by living anionic polymerization of styrenes, butadienes, and isoprenes, only very recently have synthetic methods for producing polyolefin block and graft copolymers emerged. In the past decade, a variety of “living” alkene polymerization chemistries have been identified, which suppress chain termination and transfer processes so that precise control of molecular weight, molecular architecture, and stereochemistry can be achieved2. By means of these catalyst systems and procedures, it is now possible to synthesize a wide variety of semi-crystalline polyolefin block and graft copolymers. For example, triblock copolymers with a linear A-B-A architecture have been prepared in which the A blocks are semicrystalline and either iPP or sPP, and the B blocks are amorphous (non-crystalline) statistical copolymers of ethylene and propylene with a low glass transition temperature (i.e. EPR). Such materials have been shown to have excellent elastomeric properties that are unique among thermoplastic polyolefins.3 
Arriola and co-inventors from Dow Global Technologies (“Dow patent”) disclose a commercially practical system for the preparation of statistical multiblock copolymers consisting of semicrystalline PE blocks alternating with poly(ethylene-r-1-octene) amorphous rubbery blocks4. The process involves the use of two single-site catalysts, one for each type of block, and a “chain shuttling agent”, which facilitates exchange of growing polymer chains between the two catalyst sites. The Dow process does not rely on living olefin catalysts to create controlled architecture block copolymer materials; rather the statistical nature of the chain shuttling and termination/transfer processes yields block copolymer materials with a distribution of block molecular weights and number of blocks per chain. Nonetheless, it is disclosed that the ethylene-octene statistical multiblock polymers have thermal and mechanical properties that are superior to conventional ethylene-octene random copolymers for applications as thermoplastic elastomers.5 
Thermoplastic elastomers based on styrenic block copolymers (SBC) are often blended with oils, plasticizers, tackifiers, and other low molecular weight diluents to soften the materials, modify their processing, rheological, and adhesive properties, and to lower their cost. Such “oil extension” produces soft thermoplastic elastomeric materials that can be categorized as gel compositions. Chen of Applied Elastomerics Inc. discloses that gel compositions based on SBCs that contain one or more semicrystalline PE blocks have improved tear resistant properties6. However, the Chen patent does not claim gel compositions produced from polyolefin block copolymers.
The Dow patent mentioned above4 discloses the compounding of polyethylene-poly(ethylene-r-octene) multiblock copolymers with oil to create elastomeric gel compositions that are soft, yet have better heat resistance than gel compositions from a comparison ethylene-octene random copolymer. However, the patent does not disclose the use of mechanical or thermomechanical processing to improve the strength and recoverable elasticity of such polyolefin block copolymer/oil gel compositions, nor does it disclose the use of oils or other diluents as a processing aid that are later removed to create new high strength elastomeric compositions that have an initial elastic modulus at strains less than 1, an elastic tangent modulus at large strains just prior to break, the large strains preferably being greater than 5, the ratio of the tangent modulus prior to break to the initial modulus exceeding 50, and excellent elastic recovery with more than 70% of the large strain deformation recoverable.
In 1979, P. Smith and coworkers7,8 reported that high molecular weight polyethylene (PE) homopolymer can be processed in dilute solution using a diluent such as decalin to produce a monofilament gel fiber. Subsequent mechanical extension (drawing) of the gel fiber and removal of the diluent by means of a thermal treatment produced highly crystalline fibers with ultra-high strength and high modulus. However, this work did not involve compositions containing polyolefin block copolymers, nor did it produce elastomeric materials.
In the research that led to this invention, we made the unexpected discovery that the gel processing technique developed by P. Smith and coworkers, which had heretofore only been applied to create rigid ultra high-strength fibers and film, could be used in connection with polyolefin block copolymers to create unique elastomeric materials that have a low initial modulus at strains less than 1, a large tangent modulus at large strains just prior to break (those strains exceeding 5), the ratio of the tangent modulus prior to break to the initial modulus exceeding 50, and excellent elastic recovery with more than 70% of the large strain deformation recoverable.